Sack Lunch Seminar (SLS)

We quantify the relationship between the location of the intertropical convergence zone (ITCZ) and the atmospheric heat transport across the equator (AHTEQ) in climate models and in observations. The observed zonal mean ITCZ location varies from 5.3oS in the Boreal winter to 7.2oN in the Boreal summer with an annual mean position of 1.65o N while the AHTEQ varies from 2.1 PW northward in the Boreal winter to 2.3 PW southward in the Boreal summer with an annual mean of 0.1 PW southward. Seasonal variations in the ITCZ location and AHTEQ are highly anti-correlated in the observations and in a suite of state of the art coupled climate models with regression coefficients of -2.7 and -2.4 o PW-1 respectively. We also find that seasonal variations in ITCZ location and AHTEQ are well correlated in a suite of slab ocean aquaplanet simulations with varying ocean mixed layer depths. However, the regression coefficient between ITCZ location and AHTEQ decreases with decreasing mixed layer depth as a consequence of the asymmetry that develops between the winter and summer Hadley cells as the ITCZ moves farther off the equator.

We go on to analyze the annual mean change in ITCZ location and AHTEQ in an ensemble of climate perturbation experiments including the response to CO2 doubling, simulations of the Last Glacial Maximum, and simulations of the mid-Holocene. The shift in the annual average ITCZ location is also strongly anti-correlated with the change in annual mean AHTEQ with a regression coefficient of -3.2 o PW-1 similar to that found over the seasonal cycle.